Overhead projectors are well known, and are widely used in teaching, in sales presentations, and in other applications in which a visual display is desired.
Overhead projectors generally comprise a glass table or horizontal support with a light underneath, and with an overhead optical system including a mirror and a projection lens. A transparency is laid on the glass table, and an enlarged image of the transparency is projected onto a vertical display screen or the like.
The person with the projected image generally wishes to take it up step by step to present information in an orderly sequence. Unfortunately, many viewers of the display, whether they be prospective purchasers or students, tend to read the material faster than the presenter wishes. Thus, at least some of the viewers do not receive the visual material and the oral lecture at the same time.